1. Field of the Invention
This invention relates to a non-aqueous electrolyte cell in which an unit cell is housed in an exterior packaging material comprised of a laminated film.
2. Description of the Related Art
Recently, the tendency is towards cordlessness and portability of an electronic equipment, primarily exemplified by a notebook type personal computer, and small-sized lightweight portable electronic equipments are being developed one after another. With increasing diversity of the electronic equipment, power consumption is increased, such that a demand for a high-capacity cell, as an energy source of the electronic equipment, in particular the secondary cell, is increasing.
Among the secondary cells, used up to now, there are a lead storage cell and a nickel/cadmium cell. As new secondary cells, nickel/hydrogen cell or lithium ion cell, are being actually used. These secondary cells, however, use a liquid as the electrolyte, and hence suffers the problem of liquid leakage from the cell.
For overcoming this problem, a polymer lithium ion secondary cell, employing a high molecular gel swollen by the electrolytic solution as the electrolyse, has been devised. With the development of the polymer lithium ion secondary cell, the problem of liquid leakage from the cell is eliminated to allow to realize a small-sized and lightweight thin-type secondary cell having a high energy density.
The structure of the polymer lithium ion secondary cell is now explained. On a positive electrode current collector, constituted by a thin aluminum sheet, an active material composed of, for example, LiCoO.sub.2 and graphite, is layered to constitute an electrode. On a negative electrode current collector, constituted by a thin copper sheet, an active material composed of, for example, carbon, cokes or graphite, is layered, to constitute another electrode. In-between the electrodes, there is arranged a separator, as a thin porous film. In-between the electrodes and the separator, a high molecular gel-like electrolyte, such as polyacrylonitrile (PAN), polyethylene oxide (PEO) or polyvinylidene fluoride (PVDF) is charged to complete a unit cell of a sandwich structure.
The unit cell, having the sandwich structure, is encapsulated in an packaging material, as an packaging vessel, comprised of a metal thin film, such as an aluminum foil, and a plastic film of, for example, nylon, polyethylene, polypropylene or polyethylene terephthalate.
In general, this type of the cell is loaded in a small spacing of the electronic equipment where component parts are integrated to a high packing density, and hence an exterior packaging material exhibiting high flexibility to all sorts of stress is desirable. The present inventors have already developed a method for manufacturing a card type cell in which the unit cell is encapsulated by a highly flexible packaging material while the cell characteristics are maintained (Japanese Laying-Open Patent H-8-83596).
There is also disclosed in Japanese Laying-Open Patent H-8- 101356 a technique in which an insulating layer having superior barrier characteristics with respect to the electrolytic solution is interposed between a heat-sealing plastic layer as the innermost layer and a metal layer of the packaging pouch to improve the packaging properties for the electrolytic solution.
However, the more the number of layers of the layered structure of the exterior packaging material, as in the method described in the Japanese Laying-Open Patent H-8-101356, the lower tends to become the reliability in the hermetic sealing performance of the cell, due to, for example, gas intrusion from the bonding interface of respective layers or exfoliation of the interfacial bonding portions.
Since an adhesive layer with a thickness of a least 5 .mu.m is required for bonding the respective layers, the entire layer thickness is increased, thus increasing the amount of the gas transmitted from the inner layer in its entirety.
It has, therefore, been desired to develop a thin cell packaging material exhibiting high hermetic sealing and bonding properties.